Known portable devices for warning drivers of temporary hazardous road conditions provide either a visual warning only, or if an attempted audible or mechanical warning device is provided it is of such a complicated and expensive structure that the cost tends to be prohibitive. Furthermore, such audible or mechanical road warning devices are subject to movement or "dancing" across the highway road surface during co-action with the vehicles and are therefore uncertain as to their placement retention on the road surface.
One example of a known warning device is that of a plurality of parallel, transversely extended mounds of bituminous or like material. This type of warning device has several disadvantages, however; one being the length of time of installation and or removal. Another is the permanent disfigurement of the road surface upon removal, requiring at times another process of repair in the nature of patching. Yet another is the fact that the bituminous material is then normally discarded.